1. Field of the Invention
The present invention relates to a multistage interference canceller to be used in Code Division Multiple Access (CDMA) communication systems. More particularly, the present invention relates to digital mobile radio communication systems that use a Direct Sequence Code Division Multiple Access (DS-CDMA) communication system. More particularly, the present invention relates to a method of tentative decision in the multistage interference canceller.
2. Description of the Related Art
The CDMA communication system is used for digital mobile radio communication systems in various countries. In this system, it is very important to improve the Signal-to-Interference Ratio (SIR) to decode received signals much more accurately. SIR can be adversely affected, for example, by interference from other users caused by the correlation between spreading codes.
The multistage interference canceller, which generates and removes interference replicas from received signals in multiple stages, is generally expected to improve SIR.
FIG. 1 shows a configuration example of a conventional multistage interference canceller. In this example, stages 1 to m are linked longitudinally. Each stage has interference canceller units (ICU) (71) and synthesis units (72). The subscripts attached to the names of interference canceller units (71) ICU.sub.1, 1.about.ICU.sub.1,k, ICU.sub.2, 1.about.ICU.sub.2,k, . . . , and ICU.sub.m, 1.about.ICU.sub.m,k correspond to stage numbers "1" to "m" and user numbers "1" to "k."
In stage 1, received signal R.sub.0 is input to interference canceller units ICU.sub.1,1 to ICU.sub.1,k (which correspond to users). The interference canceller units then output interference replica signals S.sub.1,1 to S.sub.1,k and estimated interference residual signals d.sub.1,1 to d.sub.1,k. The synthesis unit (72) synthesizes estimated interference residual signals d.sub.1,1 to d.sub.1,k, removes them from received signal R.sub.0, and then outputs an error signal e.sub.1.
In stage 2, error signal e1 from the synthesis unit (72) in stage 1 and interference replica signals S.sub.1,1 to S.sub.1,k from interference canceller units ICU.sub.1,1 to ICU.sub.1,k in stage 1 are input to interference canceller units ICU.sub.2,1 to ICU.sub.2,k. Next, the interference canceller units output interference replica signals S.sub.2,1 to S.sub.2,k and estimated interference residual signals d.sub.2,1 to d.sub.2,k. The synthesis unit synthesizes estimated interference residual signals d.sub.2,1 to d.sub.2,k, removes them from error signal e1 input from stage 1, and then outputs error signal e2.
Similarly, in stage m, error signal e.sub.m-1 (from the synthesis unit of the previous stage) and interference replica signals S.sub.m-1,1 to S.sub.m-1,k (from the interference canceller units of the previous stage) are input. The interference canceller units in stage m then output interference replica signals S.sub.m,1 to S.sub.m,k and estimated interference residual signals d.sub.m,1 to d.sub.m,k. Thus, the interference replica signals from which the interference between users and the multipath interference is removed can be obtained by the processing in each stage.
FIG. 2 shows the configuration of each interference canceller unit (71) shown in FIG. 1. In this example, the interference canceller unit has a three-finger structure for rake (RAKE) synthesis. In FIG. 2, "81" indicates a despread unit, "82" a synthesizer, "83" a decision unit, "84" a spreading unit, "85" a synthesizer, and "86" a despreader. Also, "87" indicates an adder, "88" a multiplier, "89" channel estimation, "90" a multiplier, "91" an adder, and "92" a spreader. In the following explanation, symbol " " represents an estimated value and symbol "*" represents a complex conjugate number.
Error signal e.sub.m-1 from the previous stage (received signal R.sub.0 if stage m is stage 1) and interference replica signal S.sub.m-1,k from the previous stage (zero if stage m is stage 1) are input to the despread unit (81) corresponding to the delay profile (path) of the received signal. The despreader (86) demodulates error signal e.sub.m-1 input from the previous stage in reverse according to a spreading code by despread. Note that in stage 1, received signal R.sub.0 is input to the interference canceller in synchronization with the spreading code.
The signal spread in reverse and demodulated by the above despreader is added to the interference replica signal input from the previous stage by the adder (87). Received symbol vector R.sub.i is then generated for path i. Received symbol vector R.sub.i for path is input to channel estimation (89), which outputs an estimated value .xi.i of the channel (phasing vector) of path i. Channel estimation (89) estimates the value by using a pilot symbols included in the received signal. For instance, the estimated value of the phasing vector may refer to an error in the signal phase or amplitude caused by phasing in a radio channel.
The multiplier (88) multiplies received symbol vector R.sub.i by using complex conjugate number .xi.i * of estimated channel value .xi.i for weighting and phase compensation in proportion to the amplitude of estimated channel value .xi.i . The synthesizer (82) synthesizes the signal output from the multiplier (88) corresponding to the path at the maximum ratio.
The decision unit (83) temporarily evaluates synthesized received symbol vector .SIGMA. R.sub.i.xi..sub.i *. The synthesizer (83) outputs estimated information symbol vector Zs following hard decision of synthesized received symbol vector .SIGMA. R.sub.i.xi..sub.i *.
Output estimated information symbol vector Zs is input to the spreading unit (84). The multiplier (90) multiplies estimated information symbol vector Zs by estimated channel value .xi.i to generate interference replica signal S.sub.m,k for each path, then outputs the generated signal to the next stage.
The adder (91) subtracts interference replica signal S.sub.m-1,k from interference replica signal S.sub.m,k for each path, then outputs the result to the spreader (92). The spreader (92) despreads the signal input from the adder (91) according to the spreading code, then outputs the spread signal for each path to the synthesizer (85). The synthesizer (85) synthesizes the signals input from the synthesizer (85) and outputs estimated interference residual signal d.sub.m,k.
As the above operation is executed for users in more stages, error signal em becomes closer to noise only, resulting in higher interference replica signal accuracy. Thus, a received signal (from which the interference between users and multipath interference is removed) can be obtained after rake reception processing using the error signal and interference replica signal in the final stage.
FIG. 3 shows an example of the signal space where the decision unit (83) executes hard decision of the received signal to which QPSK modulation is applied. FIG. 3 shows in detail the first quadrant of the signal space enclosed by Q channels. In this example, received symbol vector .SIGMA. R.sub.i.xi..sub.i * is subject to hard decision to confirm that it is estimated information symbol vector Zs . The phase of received symbol vector .SIGMA. R.sub.i.xi..sub.i * is compensated so that it becomes a normal vector signal. Estimated information symbol vector Zs is then output as the signal after tentative decision at a level equivalent to the total amplitude of estimated channel value .xi.i .
As previously described, the interference canceller unit corresponding to each user of a conventional multistage interference canceller has a decision unit (83). The decision unit (83) inputs and evaluates received symbol vector .SIGMA. R.sub.i.xi..sub.i * output after rake synthesis by the synthesizer (82). The decision unit (83) evaluates estimated information symbol vector Zs (as shown in FIG. 3) even when the signal level is abnormally high or low or the phase difference is large. The difference in phase or level occurs, for example some channel estimations use known pilot symbols which are periodically inserted every information data to estimate channel by interpolating the pilot symbols, to estimate value .xi.i . The estimated value is also applied to information data and is not always optimum for every received signal due to the varying influence of phasing and noise upon received signals. The influence of phasing and noise can cause received symbol vector .SIGMA. R.sub.i.xi..sub.i * to differ significantly in level and phase from estimated information symbol vector Zs obtained by hard decision. If this occurs, the reliability of estimated information symbol vector Zs is lowered. If a less reliable estimated information symbol vector Zs is used to output interference replica and estimated interference residual signals to the next stage, the lowered capability of removing interference will cause a problem.